This invention relates to the MRI (magnetic resonance imaging) technology which is used mainly for medical diagnosis, and forms an image consisting of MR information on a selected slice by using a computer tomography, and more particularly to an MRI system which can exactly and readily select the slice position.
In the MRI system, a static magnetic field is uniformly applied to an object under inspection. At the same time, a gradient magnetic field, superposing the static magnetic field, is applied to the same. With the application of these magnetic fields, the object is applied with a selective excitation pulse of an electromagnetic wave with a resonance frequency (Larmor frequency) which corresponds to the magnetic field of a specific atomic nucleus. As a result, a magnetic resonance (MR) is selectively excited at the slice as defined by the static magnetic field and the gradient magnetic field. To excite the MR, a high frequency excitation pulse other than the select excitation pulse is often used. In most cases, the high frequency pulse is used in an additional manner. A signal generated at the slice by the MR is detected as an MR signal. In detecting this MR signal, a gradient magnetic field existing along the slice is applied, providing the location information at the slice. The MR signals thus obtained are used for forming an image based on the MR information.
In usual systems, the gradient magnetic field, which is superposed on the uniform static magnetic field Ho, is defined by gradient magnetic fields Gx, Gy and Gz in the x, y and z directions of the three dimensional rectangular coordinates. A gradient magnetic field of a desired direction can be composed by appropriately and selectively combining these gradient magnetic fields Gx, Gy and Gz. In other words, an intensity ratio among the gradient magnetic fields Gx, Gy and Gz defines a gradient angle of the slice to be selectively excited. By changing the frequency of the excitation pulse Hl, a position existing in the magnetic field direction (usually z-axis direction) of the slice on the same atomic nucleus can be selected. Thus, according to the MRI system, the MR image of the slice can easily be picked up in various directions and at various locations.
The MRI system capable of tomographing a proper slice is very useful, but the selection of a desired slice is not easy. More specifically, this system can take a tomogram of a desired, three-dimensional image. However, it is very difficult to exactly obtain a specific position of the slice to be picked up, and also the positional relation between this slice and the tomographed slice.
For the above reason, in tomographing the object by the MRI system currently used, a target portion of the object to be tomographed is roughly picked up, and its near portion is tomographed many times to have many tomographs under various conditions. From those tomograms, a desired one is searched.
Such tomographing operation needs many unnecessary tomographing operations including retomographings. This is time-consuming and troublesome work, and frequently applies heavy mental and physical load to the object, e.g. a patient.